watering

The increase in scale that has been taking place over the past ten years calls for a different way of watering. Including on the technical side – after all, how do you make sure all the plants in those long rows are getting enough water? Can you even get it all the way round? Systems are being designed for pressure loss and pumping capacities are being ramped up. What’s more, growers are opting for drip hoses with a smaller diameter. And all with the aim of getting the water and the nutrient solution to the plants faster.

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The range of plant invigorators and water treatments on offer has increased significantly in recent years. Some look set to deliver growth effects in practice.

This has sparked interest in growth tests that highlight differences in plant growth in standard conditions. There are many different growth tests available, ranging from lab tests in Petri dishes to growth tests on a semi-practical scale. We have adapted one of these – the BioTest Phytotoxkit test – into a quick, relatively cheap and broad growth response test.

The test consists of a three-day germination test with three different crops, usually mustard, cress and sorghum. The test can detect minor differences with a high degree of certainty. Other benefits are the reports on both above- and below-ground growth, and the fact that effects due to differences in nutrients, pH and EC are excluded.

The test is too short in duration to test growth effects caused by microorganisms in the root environment, but it is suitable for determining growth inhibition caused by substrate and drain water. Separate growth tests are available for measuring the growth effects of microorganisms in the root environment.

Greenhouse horticulture will have to be virtually emission-free in ten years’ time. By 2027, European growers must ensure that their drainage water is completely free from residues of fertilisers and crop protection products. Researchers are currently working on a cultivation method that involves the plant finishing up all the nitrate and phosphate in the substrate slab. But a lot depends on achieving the right balances.

A layperson may think it’s easy to grow crops with zero emissions. Just stop fertilising a few weeks before the end of the growing period so that the plants use up the last of the fertiliser – and voilà: the slab is clear, the water is clean and the greenhouse is producing emission-free.
Of course, that’s not what actually happens in capital-intensive greenhouse horticulture. The crop has to continue to produce, preferably right up until the very last day – the day before the crop is cleared. So it needs a good supply of nutrients right up until then.

The right level

“The main aim of an end-of-cultivation strategy is to end the growing and production season emission-free, i.e. with no nitrate and phosphate left in the slab. But the plant needs to continue to receive enough nutrients and water right up until the penultimate week of cultivation,” says researcher Chris Blok of Wageningen University & Research in the Netherlands. “You can quite safely stop providing nutrients in the very last week and leave the plant to fend for itself after that. But in the four or five weeks leading up to that, it’s important to maintain feeding and water levels. That way you will keep production and quality at the right level right up until the end.”

Targets and settings

Blok and his colleagues Romain Leyh and Marco Bustamante are working on a method that entails gradually reducing the supply of nutrients, but in such a way that the plant doesn’t run short of anything. Their research, which is being done on sweet peppers, is being funded via the Dutch Foundation for Water Board Research. A number of companies, including substrate specialists Grodan, are involved in it.
To begin with, the researchers set target values for week -5, week -3 and week -1. The negative week number indicates the number of weeks before the end of cultivation. To achieve the target values, the researchers chose settings for the nutrient supply. An example based on nitrogen supply: 1) The target value for week -5 is 15 mmol N/litre. 2) This should have fallen to 10 mmol/litre by week -3. 3) By week -1, the slab should only contain 5 mmol/litre. 4) So in week 0, when cultivation ends, the slab will contain no more N at all.
To achieve this, the set N supply must decrease: 1) from 15 mmol N per litre in week -5, 2) to 5 mmol/litre in week -3, 3) and finally to 1 mmol/litre in week -1. The researchers have developed similar schemes for phosphate (P), EC and water content in the slab.

Results needed fast

Can it work? Will it mean the grower ends up with a nutrient-free slab? The answer is yes, but the method requires the grower to be very vigilant. Blok: “You will have to take measurements in the slab regularly to keep an eye on how things are going. For example, you might choose a target value that is too ambitious, which would mean that you are supplying too few nutrients. The plant would then run short and production would suffer. If that happens, you would need to be able to make adjustments quickly: you can’t wait a week for the results of the measurement to arrive, you would need them within a couple of days. In the trial department we work with electronic meters which give us the water and EC results we need fast.”

Too much versus too little

As stated, all elements the plant needs must be present in sufficient – but never excessive – quantities until just before the end of production. Researchers Leyh and Bustamante: “To check that, we measure the nutrient levels in the slab twice a week. We focus mainly on whether N and P are being used up because these elements are banned from being discharged in the drain water. If the measured value differs from the target value by more than 25% – up or down – it is important to adjust the settings. Because if the measured value is too high, you won’t get the slab clear in time. If the value is too low, the crop will run short of nutrients.”

Souring the slab

In a departure from general nursery practice, the researchers are using chloride as an anion in their trials. They have to do this because as the amount of nitrate supplied drops, the supply of cations, particularly potassium, needs to remain slightly higher. So instead of providing nitrate, they supply chloride as an anion.
Another strategy that helps to clear the slab is to play around with the pH value. If you keep the pH value deliberately low with acid and ammonia, the slab turns acidic. This releases the last residues of precipitated phosphate, causing the plant to use up this nutrient as well. Blok: “This way you can get the plant to finish up all the leftovers, as it were.”

Precisely the right balance

Researcher Ruud Kaarsemaker of Groen Agro Control was involved in the research. He calculates the plant’s nutrient uptake and advises the researchers so that they can input this information alongside the target values and settings. Kaarsemaker points out that nutrient uptake should remain stable for as long as possible. “The plant should continue to grow and produce. But as the amount of food and water the plant receives decreases, the system and therefore the plant become more and more susceptible to disruptions. So it’s a question of looking for precisely the right balance.”
This method is not yet ready for use in practice. However, what is clear is that the nutrient and water supply needs to be adjusted in such a way that the slab is actually empty by the time cultivation finishes. Aspects such as weather influences also have to be taken on board in the targets and settings.
According to Blok, the method should also be highly automated, otherwise it will be too much work for the grower. “An attractive aspect of this method of working is that it doesn’t require any investment on the part of the grower. The result is achieved entirely from controlling the nutrient flow. We are essentially telling the plants: ‘It’s time to start clearing your plate’.”

Summary

Researchers at Wageningen University & Research and Groen Agro Control in the Netherlands are working on a nutritional regime in which the amount of feed and water given to the plants gradually decreases towards the end of cultivation. This forces the plant to finish up everything in the slab, thus making cultivation completely emission-free. The trick is to adjust the amount of nutrients given to ensure that production and product quality remain at the required level.

The adage that every downside has an upside even applies to the regulations on discharges, which are becoming ever stricter. Full recirculation in sweet pepper crops is now achievable in the lab, and apart from providing a much better understanding of the crop, it can also save money. Growers can already largely avoid emissions without investing in expensive cleaning equipment.

The trial department at the Water Innovation and Demonstration Centre (IDC) in Bleiswijk, the Netherlands, may be only 150 m2 in size but it produced a lot of new information in 2015. In the first trial year the researchers compared zero-emission recirculation in a sweet pepper crop grown on stone wool with conventional cultivation and discharging. A consortium of nine suppliers, the Dutch water boards, the province of South Holland and the Dutch government, are not only financing the research but are also supplying new knowledge and techniques. “Stronger together” is certainly true in this case.
After the successful results achieved in the first trial year, a second trial was run last year, this time with stone wool and coco and with zero emissions. Why coco? Eelke Hempenius of Grodan and Erik van Os of Wageningen University & Research explain: “Following on from the stone wool trial, we also wanted to test other, non-inert substrates. Basically, we need to get a complete picture of zero emission growing.”

Good irrigation water

Coco naturally contains high levels of sodium chloride. At the start of the new season, the drain water already contained 3 mmol/l sodium, compared to less than 1 mmol/l in stone wool. Therefore, the researchers filled the coco slabs with CaNO3 (EC 3.5) for the sodium and calcium exchange and the slabs were buffered.
By always using clean, low-sodium irrigation water throughout the crop cycle, they found that they could obtain almost the same yields with coco substrate as they had with stone wool. The difference was ultimately 6% in favour of stone wool. With the buffered coco slabs, a total of 8 kg nitrate per ha was discharged over the whole crop cycle, excluding the residual water and the slabs. This is a very good result, considering that the total for conventional cultivation in 2015 was 153 kg per hectare.
This positive result provides food for thought. Would it be possible, for example, to meet the discharge requirements without having to spend money on expensive systems? Yes it would, the researchers believe. Hempenius: “Good, low-sodium irrigation water is essential. If you have very good quality water, you don’t actually have to worry about what happens on the backside.” You will need plenty of rainwater storage for that clean water: 1,500 m3 per ha is the recommended amount. Reverse osmosis is also a useful addition.

No need to rinse

Traditionally, growers have drained off the first slab water before planting the crop, in the belief that it contains substances that are detrimental to the young plants’ roots. But according to Hempenius, that isn’t necessary. “Our emission tests have proved this. I have noticed that this idea is still very much ingrained in people’s minds.”
Ingrained – that’s an expression that crops up regularly. The researchers often hear growers saying they prefer not to recirculate because it means the plants are constantly getting the same nutrient solution. That’s an argument Van Os dismisses out of hand. With 30% drain, for example, 70% of the nutrient solution is refreshed, so the plants always get plenty of new nutrients.

Concerns about sodium

By far the biggest concern growers have is that sodium will accumulate in the process water. Sweet pepper is known to be able to tolerate up to around 6 mmol/l sodium, but the figure may be even higher, the researchers claim. Salt tolerance is an area that is not yet fully understood and new research is needed on it. “A lot of what we do is still based on instinct and we tend to err on the side of caution,” Hempenius believes.
This 6 mmol/l sodium level increases the EC by 0.6. With an EC of 2.5, that still allows enough leeway to create a well-balanced nutrient solution. Van Os: “So starting to drain at 4 or 5 mmol/l sodium is really not necessary.”
The nutrient solution must be matched to the plant’s needs, however. During the trial the process water was analysed once a week. In the laboratory of partner Groen Agro Control, uptake was also analysed weekly, based on irrigation, drain, temperature, light sum and CO2. This shed more light on the plant’s needs. This intensive method of monitoring resulted in a much better match between supply and uptake of nutrients.
Van Os: “By doing this, you also gain in terms of growth and possibly even yields, because you have a better picture of what the plant needs.” He also expects things to move in that direction in practice. Growers who currently only take samples every two or three weeks could substantially increase their sampling frequency. This would result in more growth, less discharge and ultimately a better understanding of the crop, eliminating imbalances in the nutrient solution.

Using up the last residues

In the research into crops grown on stone wool, a great deal of attention was paid to the end of the crop. The aim was to reduce residues from 50 to 20 m3 per ha, without discharging and with no nitrate or phosphate in the residual volume. For example, the total amount of irrigation water was gradually reduced based on the radiation sum.
The nutrient solution also had to be adjusted so that the plants would use up the last remaining nitrate and phosphate residues. This was done by replacing one-third of the nitrate with chloride. The pH was lowered to keep phosphate and trace elements available to the plant. Van Os: “This has to be done very precisely, because you can’t let the crop wilt. Tomato fruits stay firm for quite a while but with sweet pepper, the fruits tend to wilt more quickly than the crop.”
This reduction strategy doesn’t apply to coco. With this substrate, you have to drain to maintain a good nutrient balance right up until the end of the crop.

Affordable solution

Proof that recirculation with very low levels of discharge is possible is not the only insight this research has yielded. For example, the researchers have learned more about the composition of the nutrient solution and have made progress in identifying the plants’ actual needs for nutrients right through the growing season. They have also shown that sweet pepper growers can make substantial savings on water and fertilisers, ranging from five to ten percent of their total water consumption, equivalent to between 400 and 800 m3 per year. This can deliver savings of €2-€3 per m2.
By far the greatest gain can be achieved by reducing total emissions by reusing the first water released when the slabs are pierced and from the first drain. During cultivation, smart filter technologies, such as a flat-bed filter, and efficient watering, including the use of a fast ring main, help reduce the total amount of process water. Making these adjustments can save growers from having to purchase expensive cleaning equipment.
And if they do ever need to discharge drain water, they can collect it in a silo and perhaps bring in a contractor to purify it. So compliance with the new regulations doesn’t have to be a costly exercise.

Summary

Having grown sweet pepper on stone wool with zero emissions in 2015, researchers trialled coco and stone wool last year. Stone wool produced good results, with coco also heading in the right direction. What’s more, with a smart watering strategy it is possible to reduce emissions to such an extent that there is no need to invest in costly cleaning equipment. Growers still need to be willing to accept that this is a safe growing method.

Contrary to what was assumed in the past, recent research has revealed that water is rarely distributed homogeneously through the root ball following limited watering. Small changes in the composition of the potting soil or the watering regime can also have surprisingly major consequences. “So it’s all the more important for growers to take a closer look at the water uptake behaviour of their substrate and measure it routinely,” specialist Hans Verhagen says.

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The variation in mineral emission between different chrysanthemum growers is large. That is partly due to decisions made by growers. Yet to be able to steer the crop to have minimum emissions and maximum product quality good tools are required. A better moisture sensor would be welcome, says Dutch grower Jos Ammerlaan.

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Dutch tomato grower Kees Stijger, of Honselersdijk, believes that measuring the water content in a single substrate slab is not enough. Instead, he feels that a new wireless unit for measuring moisture, with its multiple sensors spread over several watering sections, gives a more representative picture of his entire crop. He grows red, orange and yellow medium-sized tomatoes on stone wool on 2.5 ha.

To understand the distribution of the water content in the Cultilene Optimaxx stone wool slabs the tomato grower uses the new module RootView from Klimlink, an analysis package for climatic data by Klaver4ICT. In a cut section of slab, the colours relates to the water content so you can see at a glance the distribution of the water in the slab.
To know the actual water content, measurements need to be taken in the stone wool slab. For this purpose the greenhouse has four wireless measuring units each with five sensors. Every sensor has three equally long probes, which are pushed into the stone wool slab at different heights and places, also under the stone wool block and in the side of the slab. Each slab has 15 different measuring points for moisture level, temperature and EC. "This gives a good picture of what is happening in the slab,” says Kees Stijger.

Analyse the measurements

The values from the slab are sent to a sensor that hangs on one of the greenhouse columns. The data that has been collected is sent via a wireless connection to a universal database. This also contains data from the climate computer, such as radiation, water supply, volume and EC of the drain water. In this way the measurements from the slab can be analysed in combination with the climate data. An average figure is taken from the sensors from the different measuring units to provide a representative and reliable picture of the water content in the slabs.
“By knowing the water level at the top as well as at the bottom of the slab the grower deduces the flow of water during the course of the day. By then playing with the water supply, the system makes it possible to reduce the drain percentage,” says Wim van Vliet, of Klaver4ICT.

Watering strategy

With these measurements the tomato grower has insight into his watering strategy. With this knowledge, he can determine the watering, such as when to start supplying water in the morning and the volume of water to give. The grower can also see the EC-gradient in the slab.
“If you look carefully at the EC, you can adjust the watering accordingly. You can allow the drain percentage to depend each day on the EC and water content of the slab. As a result you have less drain water to disinfect and possibly discharge. And that is s big advantage,” says Stijger. “Even more importantly is that the root structure remains good, so there are no problems from water shocks: you prevent the roots from drowning. If there is a good root structure in the slab, the crop above ground is also much easier to manage. That is reflected in the crop growth and fruits."

Slab dynamics

Because additional measurements don’t automatically provide more information, the grower wanted a different form of presentation. Van Vliet set about presenting the mass of data in a simple form. In the summer of 2015 this resulted in a new module that provides insight into the distribution of the water content in the slab itself.
Van Vliet: “In a vertical cut section in the length of the slab we can see the water content in the form of different colours. The grower can see at a glance the distribution of the water in the slab. This distribution, or the ‘slab dynamics’, varies from one moment to the next and therefore is easy to show in an animation.”

Graph line water content

As well as the slab dynamics the module also shows all the relevant data from the climate computer in a 24-hour-graph. With a reading line it’s possible to point to every time the slab dynamics and the graph values are at the desired level. In the graph of the water content it is possible to show part of the slab as well as an average for the entire slab. The grower can make graphs of the average at the top of the slab, the average at the bottom of the slab, the difference between the top and bottom of the slab and the water content measured per sensor. In this way he can keep a close eye on the saturated bottom layer and the difference between the top and bottom of the slab.
The graph quickly gives the grower a complete picture for the entire day, but he can also zoom in on the slab dynamic for any given moment. The module automatically shows the values of the corresponding slab dynamics at that moment.

Conditions in the slab

Thanks to the new module, the grower receives quick feedback. Stijger: “During the spring, with the changeable weather, it’s important to know what is happening in the slab, especially with Next Generation Growing, when you use the screen more to save energy. Because there is less transpiration under a closed screen, you have to make sure that the slab is not too wet.”
The aim is to further use the data from the climate computer and the slab measurements to make separate calculations, for example, the radiation in relation to the water supply, the moisture content, the transpiration and the amount of drain. “The radiation determines the amount of water you give, but it also depends on transpiration,” says the grower.

New ideas

The module ensures that the moisture sensors don’t essentially have to be connected to the climate computer to provide a complete analysis. With the combined display of graph and slab dynamics Rootview is at the forefront of developments. If the tomato grower could decide, he would prefer to receive a graph of the water gradient via the climate computer.
Van Vliet: “As far as we are concerned, the data doesn’t have to only come from the climate computer, but data can also go to it. For example, this would offer the opportunity to make a connection that could shorten the duration of the watering. The data provided can also be used to give advice or to follow a certain strategy.”
As well as new insights the module also yields new questions and needs. For example, the grower would like insight into the possible fertiliser accumulation in the slab. The EC in the slab is measured but so far there is no form of presentation. Van Vliet is currently working on a way to visualise the EC distribution in the slab.

Summary

Dutch tomato grower Kees Stijger uses a new module that allows him to visualise the distribution of the water content in multiple stone wool slabs. This is done by showing the colour in relation to the water content in a cut section of the slab. In order to measure the water content, EC and temperature, five sensors, each with three probes, are stuck into the side of the substrate slab. With data from the climate computer such as radiation, water supply, amount and EC of the drain water, the measurements from the slab are analysed further.

Every cucumber grower has his own strategy for achieving the best yield. Frank van Lipzig wants to grow sustainably, achieve a high quality as well as harvest lots of fruits. He is now in his second season of high wire production. Last year was a transition year but now he has developed a clear strategy to keep the crop in balance.

This year Frank van Lipzig has been able to sell his cucumbers with an environmentally friendly quality mark. This is because his nursery has a good water source, supplied by a well and rainwater, and because he knows exactly what and how much residual water he drains off. The cucumbers on his 8.5 ha nursery in the ‘California’ horticultural area in Limburg in the south east of the Netherlands, look in good shape.

Less water

Last year he switched from a strategy of two cucumber crops and one tomato crop per year to two crops of cucumbers on high wires. For him it was a year of new concepts. “I decided to stop with the autumn crop due to the lack of profit from the CHP cogenerator. I also noticed that tomato growers with artificial lighting were catching me up on quality. With the high wire crop the costs per fruit are lower, I can deliver a better and more consistent quality and I have a better overview of the labour and organisation. All the different aspects just came together,” explains the good-humoured Limburger, occasionally taking a bite from a young cucumber.
Van Lipzig is very careful about water usage. Bearing in mind he wants to have the most sustainable approach possible he has also adapted his watering strategy. By perfecting the watering he wants to be able to control his crop as much as possible. “My ambition is to bring the quality to an even higher level,” he says.

Maximum result

Frank Janssen, of substrate supplier Grodan, advises the nursery about the choice of slab. Janssen: “Each grower has his own way of cultivation. The art is to match the slab with his type of plant, watering strategy and crop.”
Since 2008 the nursery has been using a 10 cm high Grotop Expert slab that lends itself to ‘Precision Growing’. His colleague, Kees Struijk, explains the concept. “It is one of our strategic decisions. It is absolutely necessary worldwide to deal sensibly with water and fertilisers because less and less is becoming available. Therefore it is a challenge for growers to achieve the maximum results with less inputs.”
Struijk has noticed a growing interest in this subject. Ten years ago the focus was completely on the climate; now controlling the crop is much more linked to energy use, choice of variety, water use and root quality.

Good control

Janssen: “We have two slabs available for cucumbers; one has a more open structure (Vital) and can be watered continuously and frequently. The other has a more closed structure, which makes it possible to have a higher desaturation capacity. Every grower has his own preference.”
Van Lipzig choose the somewhat tighter fibre structure because then he can better apply his own watering strategy. The slab has a wide control range. The water content can vary between 50 and 80% without it harming growth vigour. A Plantop Delta stonewool block, with homogenous water distribution, is placed on top of this slab. He thought for a long time about the choice of slab but left the choice of stonewool block to his plant supplier. “Everyone has their profession,” he says.
In the greenhouse the slabs are placed directly on the ground film. The dripper system in the 150 metre long path is simple. The grower uses normal labyrinth drippers and a nutrient solution is sent from both ends into the dripper hose. “I maintain good control with this combination. In principle I don’t need to water at night. During the day I can water as often I think is necessary.” During the winter that could be just one watering per 24 hours, while in the summer it’s almost continuous depending on the radiation. He doesn’t lower the EC because due to the watering strategy that’s not necessary. This remains constantly at 2.8.

Transition to high wire

“I began with this taller slab because I had a new greenhouse and therefore I expected generative crop growth,” explains the cucumber grower. In the meantime he knows everything from A to Z about the cultivating system and sees no reason to change. He has always prepared the fertiliser containers and done the watering himself simply because he enjoys it and he finds it interesting. He also gets feedback from his crop advisor. He is able to steer the crop based on a strategy of not providing any water at night and allowing the slab to dry out.
When he switched to the high wire production this strategy hardly changed. However, he did notice that cultivating a crop for longer requires a different approach to cultivating three short crops after each other. Growing a crop for a shorter period requires more attention to certain phases of the plant while a longer crop requires continuity.

Peak burden

And so he started cautiously, but nevertheless enthusiastically, with the first high wire crop. In week 9 he maintained an extra top and in so doing went from 1.5 to 3 stems per m2. “Due to this there was a real peak in plant burden which resulted in the fruits remaining on the plant for too long,” he says.
Based on this experience Van Lipzig began this season differently. This time he maintained extra tops at an earlier stage but did it in phases. In week 8 he went from 1.5 to 2.25 stems per m2 and in week 11 from 2.25 to 3 stems per m2. This suits him much better.
The grower puts all his energy into maintaining a balance in the crop, because he believes peak burdens are not the way to obtain maximum yield. “The art of growing cucumbers on a high wire is to obtain and maintain a balance in the crop.”

Summary

Cucumber grower Frank van Lipzig purposefully decided to use a stonewool slab with a tighter structure and large volume. He doesn’t irrigate at night and so allows the water volume in the slab to dry out. By using this strategy he can keep the high wire crop well balanced and achieve better utilisation of water and fertilisers.